The invention is based on a fuel injection nozzle as revealed in the ensuing specification. Injection nozzles of this general type have the advantage that the injection pump and the pressure lines leading to the injection nozzles can be designed for low pressures, because the high pressure required for injection is generated in the injection nozzles themselves. As a result of the overflow valve between the medium-pressure and high-pressure chambers in the injection nozzle, the quantity of fuel required for the next injection event passes into the high-pressure chamber as the stepped piston returns following the closure of the injection valve. In a known injection nozzle of this general type (German Offenlegungsschrift No. 27 55 222), the stepped piston and its high-pressure chamber are disposed upstream of the valve needle; the high-pressure chamber is formed in a cylindrical part inserted into the spring chamber of the nozzle housing. This embodiment requires an additional part, and depending on the embodiment of the return spring for the stepped piston may also require additional space in the axial direction of the injection nozzle.
Another known embodiment of an injection nozzle with a stepped piston (German Pat. No. 492 378) does not have a valve needle; instead, it has a valve which opens in the flow direction of the fuel, and the spherical closing member of the valve is caught along with its closing spring in a widening of the fuel conduit formed between the valve seat and the injection port. In still another known injection nozzle, which has a valve needle and is used for both preliminary and primary injection (German Offenlegungsschrift No. 15 76 478), a preliminary injection piston is embodied as a stepped piston, which is disposed laterally beside the valve needle and runs up against a stop, after which the primary injection quantity reaches the pressure shoulder of the valve needle via a bypass around the stepped piston. This embodiment would necessitate a relatively large diameter of the nozzle housing if, as in an injection nozzle of the general type discussed initially above, the stepped piston were embodied as a pressure-translating element having an effect over the entire injection stroke.